Rotating machines, such as turbomachinery, have rotating parts that are assembled within stationary parts and which must be precisely positioned therein. A rotating machine typically includes a rotatable hub (sometimes referred to as a wheel or a disk) that includes mounting slots or other cavities defined by cavity surfaces that are accessible from outside of the hub. An anchor structure (sometimes referred to as a root or blade root) of a rotatable body (e.g., a fan blade, a turbine blade, etc.) is disposed in each mounting slot or other cavity. Curved roots and straight roots (i.e., linear roots) are two common varieties of blade roots. In general, curved roots and straight roots may be dovetail roots or non-dovetail roots. In some cases, for example, for a blade that comprises an airfoil of a particular shape, a curved root may be preferable to a straight root while a straight root may be preferable in another blade design.
In designs in which a plurality of fan blades is carried by a rotatable hub of a turbofan gas turbine engine, the blade roots of the fan blades are made of a base metal coated with a DFL (dry-film-lubricant) that directly contacts the surfaces defining the mounting slots. This coating system can be worn away quickly due to high localized contact loads, resulting in frequent and costly repairs and disruptions for the customer.
Turbofan blades having straight root dovetails typically have fewer wear problems than curved root dovetails since the worst-case operating scenario for a straight root dovetail is line contact between the hub and the dovetail over the length of the dovetail. High localized contact loads may still occur for straight root designs that include axial tangs and/or high hade angles.
U.S. Pat. No. 4,169,694 discloses a curved root dovetail design with double curvature that results in a more uniform transfer of loads to the mounting slot of the hub. The patent discloses that the double curvature sets the initial point of contact low on the cross section of the dovetail curved root, which forces stresses into a tri-axial state rather than a bi-axial state. However, obtaining this condition in production is unlikely as the tolerances required to achieve this in practice are probably very tight.
U.S. Pat. No. 6,457,942 discloses that fan blades having a curved root dovetail experience point loading contact with a wheel. An axial retention feature is provided to minimize point loading in a blade failure event.